1. Field of the Invention
The present invention relates to an apparatus and method for electrophotographic image forming.
2. Description of the Prior Art
A process for electrophotographic image forming will be described with reference to FIG. 7.
A process for forming an image comprises the steps of charging, exposure, development, transferring, cleaning, fixation and charge removal. A photoreceptor drum 1 is provided in such a way that it can rotate to a direction indicated by an arrow S1. The surface of the photoreceptor drum 1 is evenly charged to a predetermined quantity of charge with charging means 2 such as a corona charger and a contact-type charging roller, and may carry an electrostatic latent image created by a predetermined electrostatic latent image potential generated by exposure means 3.
The photoreceptor drum 1 comprises a conductive substrate made of a metal or resin, an undercoating layer formed on the surface of the substrate, and a photosensitive layer formed on the undercoating layer. The photosensitive layer consists of a relatively thinner charge generation layer (CGL) formed on the undercoating layer and a relatively thicker charge transport layer (CTL) mainly formed of polycarbonate which is formed as the outer layer. In the charge generation layer, exposure generates carriers whereby a charge on the photoreceptor drum 1 is cancelled to generate the above electrostatic latent image potential.
The electrostatic latent image carried on the photoreceptor drum 1 is transported to a developing area 42 in contact with a developer carrier 41 as the drum 1 rotates. The developer carrier 41 which rotates to a direction indicated by an arrow S3 opposite to the rotation direction S1 of the photoreceptor drum 1 is pressed on the photoreceptor drum 1. Thus, a toner 10 carried in the developer carrier 41 is moved and adheres to the photoreceptor drum 1 according to the electrostatic latent image on the drum to visualize the electrostatic latent image, and thus, development is completed. A predetermined bias voltage is applied to the developer carrier 41 from an unshown power supply connected thereto.
After development, the toner 10 adhering to the photoreceptor drum 1 is transferred to a predetermined transfer region, to which a transfer material P such as a paper is supplied by a paper feeder and the transfer material is synchronously brought into contact with the toner image on the photoreceptor drum 1. The transfer means 5 provided in the transfer region may be a charger type or a contact roller type with a high voltage power supply and applies to the photoreceptor drum 1 a voltage having a polarity of a side to which the toner 10 is to be transferred. Thus, the toner 10 is moved to the transfer material P so that the toner image is transferred. After separating the transfer material P from the photoreceptor drum 1, the toner on the transfer material P is fixed by a fixing means 8. For example, the material is fixed by thermal melting and then ejected from the apparatus. The surface of the photoreceptor drum 1 after transfer is cleaned by a cleaning means 6 and the residual charge on the surface is removed by a charge erasing means 7 to electrically initialize the surface. The charge erasing means 7 includes a charge erase lamp and a contact charge eraser.
Conventionally a gas laser has been used in a copier or printer employing an electrophotographic process where line scanning is conduced with a laser beam, but a semiconductor laser has been recently used because of its reduced size and cost.
Such a semiconductor laser generally requires an electrophotographic photoreceptor with high sensitivity in a long wavelength range of 750 nm or more, and attempts have been made for developing such an electrophotographic photoreceptor.
It, however, has a drawback that laser beam exposure to a photoreceptor which is sensitive to a long wavelength light may cause interference fringes in the toner image formed, leading to poor image reproduction.
It may be partly because, as shown in FIG. 8, in a conventional laminated photoreceptor having a photosensitive layer consisting of a conductive support 11, a charge generation layer 12 and a charge transport layer 15, a laser beam enters as an incident beam into the photosensitive layer, and is then reflected at the interface between the photosensitive layer and the support and the interface between the photosensitive layer and the air as a reflected beam 21, and interface fringes are formed due to a phase difference between the reflected beam 21 and the incident beam 19.
To overcome the drawback, there have been proposed elimination of multiple reflection in a photosensitive layer by, for example, roughening the surface of a base pipe (conductive support) in a photoreceptor by anodization or sand blasting, or using a light absorbing layer or antireflection layer between a photosensitive layer and a base pipe. In practice, however, interference fringes appearing during image forming cannot be completely eliminated.
For example, Japanese Patent Publication No. 5-26191 has disclosed a technique in which irregularity on the order of 0.1 to 1.0 xcexcm is formed on a base surface.
With the recent improvement of image quality and resolution, it has been found that a resolution of 1200 dpi or more may lead to interference fringes even in such a rough surface. It might be because as the dot number in a unit area increases, reflected light is increased, so interference due to the reflected light is increased and the increased interference appears as interference fringes so that a conventional surface roughness cannot eliminate the increased interference fringes. It is, therefore, necessary to further roughen the surface of a base pipe for improving light scattering so as to deal with interference fringes associated with improvement in image quality and resolution. On the other hand, when a roughness (the maximum roughness Rmax) is excessively high in the support pipe surface, a large rough area may act as a carrier injection area to a photosensitive layer to cause a white spot (or black spot when using a reverse developing system) during image formation or appearance of the surface shape of the base pipe in an image formed. Furthermore, an excessively rough surface may cause an uneven film thickness during an application process, leading to problems in an image.
Therefore, an object of the present invention is to provide an image forming apparatus and a method for forming an image whereby problems in an image due to interference fringes can be eliminated at a higher resolution of 1200 dpi or more to improve image quality. Another object of this invention is to economically provide such an apparatus by selecting a base pipe surface roughness Rmax whereby production may be easily managed.
An image forming apparatus of this invention has a configuration wherein an electrostatic latent image is formed by exposing an electrophotographic photoreceptor having a photosensitive layer formed thereon via an undercoating layer on a conductive support with the maximum surface roughness defined by the equation:
(0.0006x+0.34) xcexcmxe2x89xa6Rmaxxe2x89xa62.5 xcexcm
where x=a resolution; visualizing the latent image with a toner to give a visualized image; and transferring the visualized image to a transfer medium.
This image forming apparatus comprises a conductive support with a surface roughness within the upper and lower limits so that it can prevent problems in an image (mainly interference fringes) in image forming for improved image quality by exposure with a resolution of 1200 dpi or more using a semiconductor laser beam.
Forming an undercoating layer (a UCL layer) between the photosensitive layer and the conductive support permits uniformly forming subsequent layers, that is, a photosensitive layer, a charge generation layer (CGL) and a charge transport layer (CTL). An area without interference fringes can be made larger than that in an apparatus without an undercoating layer. The undercoating layer may prevent deterioration in charging property during repeated use, reduce a W-charge and improve charging property under the conditions of a low temperature and a low humidity. Furthermore, in manufacturing an image forming apparatus, defining the limits of a surface roughness for a conductive support (base pipe) can allow an apparatus to be produced whereby problems in an image can be minimized, with a lower cost and easier production management.
In the image forming apparatus of this invention, the surface of the conductive support which is in contact with the undercoating layer has a different roughness depending on a resolution, whereby a high quality image with a higher resolution may be achieved.
This invention also provides an image forming apparatus wherein the undercoating layer in the photosensitive layer contains an inorganic oxide.
Thus, in the undercoating layer in which the inorganic oxide is dispersed, its resistance can be controlled and the layer may contribute to reduction of interference fringes by scattering a transmitted light. Even when the surface of the base pipe is rough enough to prevent interference fringes, the roughness may not affect the formed image.
This invention also provides a process for forming an image comprising the steps of forming an electrostatic latent image by exposing an electrophotographic photoreceptor where a photosensitive layer is evenly formed via an undercoating layer on a conductive support having a rough surface with the maximum surface roughness defined by the equation:
1.02 xcexcmxe2x89xa6Rmaxxe2x89xa62.5 xcexcm,
with a laser beam carrying an even charge and image information at a resolution of 1200 dpi or more; visualizing the latent image with a toner into a visualized image; and transferring the visualized image to a transfer medium to form an image.
According to this process, the surface roughness of the conductive support may scatter the reflected light to prevent interference fringes due to exposure with a semiconductor laser at a high resolution.
The undercoating layer (UCL) formed in the photosensitive layer in the photoreceptor drum with a rough surface allows the photosensitive layer to be evenly applied during a dip coating process, resulting in prevention of an uneven image. The undercoating layer can reduce a large rough area in the support and allows a higher upper limit to be selected for a roughness in the conductive support. Furthermore, it can inhibit causes for white spots (or black spots) in an image forming area.